The present invention relates generally to epoxy hardeners and in particular to new and useful groupings of epoxy hardener compositions which rapidly cure an epoxy resin over a range of temperatures and a method for selecting hardeners to obtain particular characteristics of hardened epoxies.
There are two general types of polymers; thermoplastic and thermosetting. Thermoplastic polymers melt on heating and solidify on cooling. They can be remelted and resolidified repeatedly within limits.
Thermosetting polymers do not melt on heating. They soften only and, if heated sufficiently will char. Thermosetting resins such as epoxies are molded by either injecting or placing by hand a prepolymer mixture consisting of epoxy resin, hardener, catalyst and various modifiers and fillers into a mold and heating for a time sufficient to complete the chemical reactions between the epoxy resin and the hardener, resulting in a thermosetting polymer having the shape and size of the mold. Molding times are considerably longer than for thermoplastic molding processes, typically five to fifteen minutes. However, injection pressures are low and so the mold clamping forces needed are also low. Thus, thermoset molding processes are characterized by low production rates and relatively low capital investment. But, large part sizes are possible.
The curing reactions of epoxies are exothermic and cure times and temperatures are determined by the heat transfer rate from the mold to the part and the scorch temperature of the epoxy. Epoxy/hardener systems which cure at low temperatures and which develop low exotherm as a result of chemical factors are very advantageous since they can be cured faster and will result in higher production rates.
For hand application, the epoxy resin and hardener are usually supplied in two separate syringes which have a common plunger. Pressing the plunger releases the correct proportions of epoxy and hardener. The two compounds are mixed with a spatula and applied to the bonding surfaces and then cured either at room temperature or at elevated temperature, depending on the application. Epoxy hardeners which cure rapidly at low temperature develop higher bond strength due to lower shrinkage stresses and permit higher production rates with lower energy expenditure.
Epoxy adhesives are frequently used in industrial processes in the form of “film adhesive”. A prepolymer mixture of epoxy, hardener, and other desired components is applied as a coating onto a polymer film substrate, rolled up and stored in a freezer to stop the chemical reactions between components. When needed, the film adhesive is removed from the freezer and applied to a metal or composite part, the backing is stripped off and the assembly completed and cured in an oven or autoclave.
At the point when the adhesive is removed from the freezer, the epoxy mixture begins to cure slowly at room temperature. After a certain time called the “out time”, the adhesive will become stiff and brittle and unusable. Latent mixtures having long out times are highly desirable in order to complete complex assemblies before curing. Hardeners having very long out times, or latency, but with relatively low cure temperatures and short cure times are difficult to create, further increasing their value.
Hardeners for epoxies are known and have been disclosed, for instance in U.S. Pat. No. 3,812,202, which teaches a two part self-hardening epoxy composition which is formed by a phenol precursor combined with a methylol acrylic polymer. The phenol precursor is made by combining bisphenol A with a polyepoxide compound to create a composition having two or more phenolic groups. The methylol acrylic polymer can be formed by polymerizing acrylamide or diacetone acrylamide with other ethylenically unsaturated monomers, followed by adding an aldehyde, such as formaldehyde, and optionally, a catalyst. The phenol precursor and methylol acrylic polymer are mixed to a desired viscosity, applied, and heated to at least about 300° F. to cure.
U.S. Pat. No. 4,866,133 discloses a curing agent for an epoxy containing a polymeric phenol and a polyamine. The curing agent is provided as a powdered latent curing agent mixed with a liquid epoxide resin. Polyamines used in the curing agent include diethylenetriamine and triethylenetetramine, among others. The polymeric phenols include different novolaks prepared from bisphenol A and formaldehyde, a novolak prepared from p-cresol and formaldehyde and a poly(p-vinylphenol), among others. The curing agent is activated by heating to at least about 60° C.
U.S. Pat. No. 5,107,036 teaches a curing agent for epoxy which is a combination of two phenol compounds. One phenol is a polyhydric phenol, formed from a condensation reaction of a phenol having at least one phenolic hydroxyl group with a hydroxybenzaldehyde compound. The hydroxybenzaldehyde used in the condensation reaction must have a hydroxyl group and an aldehyde group bonded to a benzene ring, which may be substituted with at least one other constituent. The other phenol is a dihydric phenol, such as catechol, resorcinol, and bisphenol A.
Mixtures of bisphenol A and an aliphatic polyamine are disclosed in U.S. Pat. No. 4,221,890. In one embodiment, butyl glycidylether is added to the mixture which may result in the conversion of some of the bisphenol A to a secondary polyol, as well as the formation of adducts of the polyamine with the monoepoxide. There is no appreciation for the exothermic nature of the reaction between bisphenol A and the polyamine. Further, there is no consideration of the use of methylol-functional hardeners for epoxy resins, either alone or in combination with other types of polyols.
Mannich and Schiff bases are generally known. For example, Canadian patent 591,210 to Zumstein, published Jan. 19, 1960, describes a number of Mannich bases having at least one tertiary amine group and at least one phenolic hydroxyl group as accelerators for curing mixtures of epoxies, polyamides and polyamines. The product consisting of one mole alkylphenol, one mole diethanolamine and one mole formaldehyde is also disclosed in the Canadian patent. None of the formulations described contain a polyol, however, and latency properties are not disclosed. The Mannich base consisting of one dialkylaminopropylamine group and one phenolic hydroxyl group and having unique properties as a latent hardener for epoxies has not been described. The corresponding Schiff bases are not described as latent epoxy hardener components either.
Similar products have been known for a long time, for example 2,4,6-tris(dimethylaminomethyl)phenol, which is an available industrial product. See, H. A. Bruson and C. W. MacMullen, “Condensation of Phenols with Amines and Formaldehyde”, Jl. Am. Chem. Soc., 63, 270 (1941).
European patent application EP 84301251.9 to McClain, describes an epoxy hardener consisting of a mixture of imidazole, TMP and butanediol which is soluble in epoxy and which is disclosed as curing an epoxy at 93° C.
Examples #1 and #4 of the McClain EP patent application were reproduced. But, instead of curing the samples at 93° C., an attempt was made to cure them at 65° C. These samples identified as #EP-1 and #EP-4 are described below:
#EP-1: 1.7 phr Imidazole, 0.71 phr TMP, 1.0 phr butanediol and 0.19 phr water were blended together in the order given, heating the Imidazole and TMP as required and 5 g EPON828 was then added. This sample was heated at 65° C. After 4 hrs, the sample was hard, slightly dentable with a spatula, and had a tacky surface film.
#EP-4: 1.7 phr Imidazole, 1.1 phr TMP and 2.2 phr butanediol were blended together in order and 5 g. EPON828 was then added. After 4 hrs. at 65° C. this sample was hard, slightly dentable with a spatula and had a tacky surface film.
Further testing revealed that both samples were weak and brittle after curing. Hardened epoxies having such properties are useless in the majority of applications. Thus, the epoxy hardeners disclosed in the EP application of McClain clearly require the elevated cure temperature to produce a useful result.
It can be concluded that the degree of cure of both samples was too low to develop good mechanical properties. The tacky surface film on both samples appeared to be primarily butanediol, due to the slow reaction rate of this polyol with the epoxy at the lower 65° C. temperature. Further, the poor compatibility of the butanediol with the partially cured epoxy resulted in migration to the surface and exudation, similar to the behavior of an incompatible plasticizer.
It has been shown that the degree of cure of an epoxy-hardener system is indicated by the relative proximity of the glass transition temperature (Tg) of the cured epoxy system to the ultimate glass transition temperature (Tgu) of the same epoxy system cured at a sufficiently high temperature to attain the maximum value of the glass transition temperature. The glass transition temperature Tg has no particular significance of itself in determining the degree of cure: it is only the value of Tg relative to the ultimate glass transition temperature Tgu which is significant.
Extensive experimentation with epoxy-hardener systems has shown that the curing reactions cease when the glass transition temperature Tg reaches about 40–45° C. above the cure temperature, Tc, due to viscosity effects. The maximum glass transition temperature which can be obtained is therefore given approximately by the equation: Tg=Tc+45° C.
The industries which use epoxy systems are continually pressing for lower cure temperatures and shorter cure times for largely economic reasons. In particular, high cure temperature epoxy-hardener systems require specialized ovens or other means to cure products using such epoxy-hardener systems. Since the ultimate glass transition temperature (which represents the fully cured state) can only be achieved by curing the epoxy-hardener mixture at a temperature above the ultimate glass transition temperature, it follows that all practical epoxy systems are undercured.
Clearly, there are many uses for epoxies and epoxy systems. For example, applications exist for epoxy systems in military and aerospace products, civil aircraft, sporting goods such as fishing rods, golf club shafts, tennis rackets, bows and arrows and the like, as rust preventative coatings, and potting of electronic circuits and electronics components, among others. Epoxy hardeners which can more rapidly cure epoxy without foaming or resulting in unstable compositions are desirable and useful. Hardener compositions which provide long latency periods when combined with epoxies have many applications also. And, hardeners which can cure epoxies at relatively low temperatures yet produce good mechanical properties in the hardened epoxy system are very useful.